The present invention relates generally to the field of RFID (radio frequency identification) and in particular to a new and useful plasma-based sensor array used to detect the presence of an interactive element resulting from interaction of antennas having variable conductive sections by magnetic induction and/or electromagnetic waves.
RFID systems have gained much popularity recently as a means for wireless tracking of individual objects for a variety of purposes. For example, some retailers have proposed using unique RFID tags attached to products they sell to be able to track each piece from the distribution warehouse to the store shelves, and potentially, to customer's home. RFID systems have applications in anti-theft, product marketing, intelligence gathering, and security systems, among others.
Near-field readers incorporating sensors and identification tags are generally known for use in scanning systems. As used herein, near fields exist at distances ranging from a fraction of a millimeter to a few miles, depending on frequency. The near field is defined as when the wavenumber times the distance of the range of the antenna is less than one. The far field is defined as when the wavenumber times the distance of the range of the antenna is greater than one. The wavenumber is 2π/λ.
Near-field reader systems take advantage of magnetic field interference between a powered transceiver and a powered or passive object to detect the presence of the object by receiving a return signal from the object with the transceiver.
Presently, card and label near-field readers are formed by metal loops which read data in the near electromagnetic field. In the near-field situation, for a loop antenna, the electric field is effectively zero and only the magnetic field is present. Thus, near field loop antennas use mutual inductance between active and passive loop antennas to cause the active loop antenna to receive data from the passive loop antenna. That is, the magnetic flux from one loop antenna induces a current in a second loop antenna having properties dependent on the current and voltage in the first loop. The magnetic flux interaction and induced current can be used to transmit information between the loop antennas because of the dependency. The near-field loop antennas can be more correctly considered loop sensors or loop readers, since there is no electric field interaction between the active source and a passive loop.
RFID systems, in contrast, can be both near and far field devices. RFID systems generally have a longer range than most near-field systems, because they use radio frequencies, such as 900 MHz, 2.4 GHz, and, more recently, 5.8 GHz to transmit and receive information between sensor units and passive ID tags.
A problem with all metal antennas used in a sensing array is that even when they are not active, several antennas arranged in a multiple orientation array still create unavoidable mutual inductance and electromagnetic wave interferences between antennas. That is, even if the metal antenna sensors in an array are sequentially activated, they still cause mutual interference with other ones of the antennas. The interferences result in detuning of the antennas in the array, so that special considerations must be made when forming arrays of metal antennas.
In the case of inductive loop antennas, to optimize the strength of the mutual inductance field between an active loop sensor and a passive loop antenna, the antennas must be parallel to each other. If the antennas are perpendicular, the magnetic field is zero at the passive loop and there is no mutual induction. The strength of the magnetic field at the passive loop increases as the loops move from a perpendicular to a parallel orientation. For a device to effectively scan a region for a passive loop, a single loop must move through a variety of orientations. The range of effectiveness of an antenna is based on the orientation of the passive and active loops to each other and the diameter of the loop of the active sensor.
Patents describing scanning antenna systems using interaction between active and passive antennas include U.S. Pat. No. 3,707,711, which discloses an electronic surveillance system. The patent generally describes a type of electronic interrogation system having a transmitter for sending energy to a passive label, which processes the energy and retransmits the modified energy as a reply signal to a receiver. The system includes a passive antenna label attached to goods that interacts with transmitters, such as at a security gate, when it is in close proximity to the transmitters. The label has a circuit which processes the two distinct transmitted signals from two separate transmitters to produce a third distinct reply signal. A receiver picks up the reply signal and indicates that the label has passed the transmitters, such as by sounding an alarm.
U.S. Pat. No. 3,852,755 teaches a transponder which can be used as an identification tag in an interrogation system. An identification tag can be encoded using a diode circuit in which some diodes are disabled to produce a unique code. When the identification tag is interrogated by a transponder, energy from the transponder signal activates the electronic circuit in the tag and the code in the diode circuit is transmitted from the tag using dipole antennas. The transponder uses a range of frequencies to send a sufficiently strong signal to activate a nearby identification tag.
A vehicle identification transponder using high and low frequency transmissions is disclosed by U.S. Pat. No. 4,873,531. A transmitting antenna broadcasts both high and low frequency signals that are received through longitudinal slots in a transponder waveguide. Transverse pairs in the waveguide adjacent the longitudinal slots indicate a digital “1”, while the absence of transverse pairs produces a digital “0”. The high and low frequencies are radiated from the transverse pairs to high and low frequency receiving antennas. The transmitting and receiving antennas are fixed relative to each other and move with respect to the transponder.
U.S. Pat. No. 5,465,099 teaches a passive loop antenna used in a detection system. The antenna has a dipole for receiving signals, a diode for changing the frequency of the received signal and a loop antenna for transmitting the frequency-altered signal. The original transmission frequency is changed to a harmonic frequency by the diode.
As discussed above, near-field loop sensors or readers differ from far field loop antennas by the basic difference that in the near-field, the electric field is usually very small and the magnetic field of an electromagnetic radiant source is controlling, while in the far field, the interaction is via electromagnetic waves. As will be appreciated, the relationships between sources and receivers are different as well due to the different distances and fields which affect communication between them.
Plasma antennas are a type of antenna known for use in far field applications. Plasma antennas generally comprise a chamber in which a gas is ionized to form plasma. The plasma radiates at a frequency dictated by characteristics of the chamber and excitation energy, among other elements.
Plasma antennas and their far field applications are disclosed in patents like U.S. Pat. Nos. 5,963,169, 6,118,407 and 6,087,992 among others. Known applications using plasma antennas rely upon the characteristics of electromagnetic waves generated by the plasma antenna in far field situations, rather than magnetic fields in near-field conditions.